A battery is a device that converts chemical energy into electrical energy by means of electrochemical reduction-oxidation (redox) reactions. In secondary or rechargeable batteries, these electrochemical reactions are reversible, which allows the batteries to undergo multiple charging and discharge cycles.
Secondary lithium ion batteries generally include one or more electrochemical cells having a negative electrode, a positive electrode, and an electrolyte for conducting lithium ions between the negative and positive electrodes. A porous separator wetted with a liquid electrolyte solution may be sandwiched between the electrodes to physically separate and electrically insulate the electrodes from each other while permitting free ion flow. Each of the negative and positive electrodes is typically carried on or connected to a metallic current collector. The current collectors may be connected to each other by an interruptible external circuit through which electrons can pass from one electrode to the other while lithium ions migrate in the opposite direction through the electrochemical cell during charging and discharge of the battery.
The positive electrode in a lithium ion battery typically comprises a lithium-based intercalation host material that can undergo the reversible insertion or intercalation of lithium ions. The negative electrode typically comprises an intercalation host material that can undergo the reversible insertion or intercalation of lithium ions at a lower electrochemical potential than the material of the positive electrode such that an electrochemical potential difference exists between the electrodes. The electrolyte comprises a material suitable for conducting lithium ions and may be in solid or liquid form. A suitable non-aqueous liquid electrolyte may comprise a solution including a lithium salt dissolved or ionized in an organic solvent or a mixture of organic solvents.
Lithium ion batteries can reversibly supply power to an associated load device on demand. More specifically, electrical power can be supplied to a load device by a lithium ion battery until the lithium content of the negative electrode is effectively depleted. The battery may then be recharged by passing a suitable direct electrical current in the opposite direction between the electrodes.
During discharge, the negative electrode contains a relatively high concentration of intercalated lithium, which is oxidized into lithium ions and electrons. The lithium ions travel from the negative electrode (anode) to the positive electrode (cathode) through the electrolyte. At the same time, the electrons pass through the external circuit from the negative electrode to the positive electrode. The lithium ions are assimilated into the material of the positive electrode by an electrochemical reduction reaction. The battery may be recharged after a partial or full discharge of its available capacity by an external power source, which reverses the electrochemical reactions that transpired during discharge.
During re-charge, intercalated lithium in the positive electrode is oxidized into lithium ions and electrons. The lithium ions travel from the positive electrode to the negative electrode via the electrolyte, and the electrons pass through the external circuit to the negative electrode. The lithium cations are reduced to elemental lithium at the negative electrode and stored in the material of the negative electrode for reuse.